Re: [PATCH] AArch64: Add support for -mdirect-extern-access

[Richard Sandiford <richard.sandiford@arm.com>]

Wilco Dijkstra <Wilco.Dijkstra@arm.com> writes:
> Hi Richard,
>
>> Can you go into more detail about:
>>
>>    Use :option:`-mdirect-extern-access` either in shared libraries or in
>>    executables, but not in both.  Protected symbols used both in a shared
>>    library and executable may cause linker errors or fail to work correctly
>>
>> If this is LLVM's default for PIC (and by assumption shared libraries),
>> is it then invalid to use -mdirect-extern-access for any PIEs that
>> are linked against those shared libraries and use protected symbols
>> from those libraries?  How would a user know that one of the shared
>> libraries they're linking against was built in this way?
>
> Yes, the usage model is that you'd either use it for static PIE or only on
> data that is not shared. If you get it wrong them you'll get the copy
> relocation error.

Thanks.  I think I'm still missing something though.  If, for the
non-executable case, people should only use the feature on data that
is not shared, why do we need to relax the binds-local condition for
protected symbols on -fPIC?  Oughtn't the symbol to be hidden rather
than protected if the data isn't shared?

I can understand the reasoning for the PIE changes but I'm still
struggling with the PIC-but-not-PIE bits.

> In the future we need to decide what the ABI is and
> ensure GCC and LLVM are compatible. An import feature to mark symbols
> that may be overridden by a shared library would be useful too.
>
>> It looks like the main difference between this implementation and
>> the x86 one is that x86 allows direct accesses to common symbols.
>> What's the reason for not doing that for AArch64?  Does it not work,
>> is it a false optimisation (i.e. pessimisation), or did it not seem
>> important now that -fno-common is the default?
>
> I don't see any difference in the way common symbols are accessed on x86,
> so it's not clear which cases common_local_p param actually affects (eg. with
> -fPIC there is always a GOT indirection for common symbols).

Hmm, OK.  Could it be for one of the other languages?  But yeah,
if we don't have a testcase for it, I agree it's better to leave
things as they are.

Thanks,
Richard